Water produced by various commercial and industrial processes is often contaminated, particularly by oil and solid materials as is the case in the downstream petroleum industry, or may disadvantageously contain different component phases. Prior to discharge or re-use of this produced water, some form of treatment is generally required (either to re-use the produced water in subsequent process stages, or due to governmental regulation of discharge quality).
In the context of the petroleum industry, produced water can contain small oil droplets held in suspension. Various methods and apparatus have been proposed to enable the separation of the oil from the produced water, usually employing some form of flotation system.
One separation technology employed in the petroleum industry is the use of API and gravity separation tanks, such as a “skim tank”. This technology is relatively simple and inexpensive, depending on the different densities of oil and water to enable gravity separation. Contaminated water is held in a vessel for a predetermined period of time, during which time the oil separates from the water and rises and collects at the vessel surface, allowing for skimming off of the oil. Parameters such as retention time, oil properties and inlet stream characteristics can be controlled to enhance separation, and tank dimensions are also of crucial importance. While such separators can be quite effective in removing larger oil droplets, however, they are generally ineffective in removing oil droplets of less than 50 microns (even if chemical treatments are added) and require substantial retention times.
Another well-known technology is the corrugated plate interceptor (CPI). In CPI vessels, corrugated plates are used to amplify the density differences by providing an inclined plate with a longer fluid travel path. With an inclined plate, individual oil droplets are presented with a shorter travel path to reach adjacent oil droplets, creating larger coalesced oil droplets which rise more quickly to the fluid surface. This allows for vessels with a much smaller footprint than with traditional gravity separation vessels, but it has the same limitation of being generally ineffective in removing oil droplets of less than 50 microns. In addition, chemical usage is increased and CPI vessels usually cope poorly with flow surges.
Induced gas flotation (IGF) vessels are also known in the industry, where gas is induced into the contaminated water (by means usually including eductors, sparging tubes and paddles) to more rapidly float the oil droplets out of the produced water. The oily froth is then skimmed off, sometimes by a baffle system. While IGF is one of the most prevalent technologies presently in use, it is still limited in terms of the oil droplet size that can be removed, and chemical treatment is therefore required. Also, the technology generally cannot be efficiently employed in retrofit situations.
Induced static flotation (ISF) technology is also known in the industry. This is another induced gas system, although it uses a different method of gas bubble generation than with IGF methods. In IGF systems, the bubbles are generated by mechanical means, while in ISF systems the bubbles are created by hydraulic methods. ISF vessels are usually separated into chambers, with gas introduction in each of the chambers, and ISF methods can be employed with a pressurized vessel. One limitation of ISF systems is that they have difficulty coping with oil concentrations above 300 ppm. In addition, such systems do not adequately address flow rate fluctuations, and retrofit capability is generally absent.
Hydrocyclones have also been used in the petroleum industry to treat contaminated produced water. These are conical tubes, and contaminated water is tangentially introduced into the upper end. The fluid spins around the tube, creating a centrifugal force that forces oil upwards and out of the tube while allowing the cleaned water to drain downwards. However, hydrocyclones are used in groupings based on flow rate, and the system cannot cope well with flow rate changes and the resultant fluid velocity shift. Also, there is a substantial pressure drop across the system, and a separate system is required to remove any solids in the produced water. Solids blockages can also be a problem, and the solids themselves can result in significant wear of the tube interior.
Finally, centrifuges have been used to treat produced water, with spinning forcing a separation of the oil and water. Unlike the hydrocyclones, centrifuges use moving parts to generate the spinning motion. While very effective in removing solids and enabling oil/water separation, the low flow rates and susceptibility to wear are problematic.
What is needed, therefore, is a water treatment vessel and method that does not require a substantial retention period, that can remove oil droplets less than 50 microns in diameter, and that can handle flow rate fluctuations. In addition, it would be advantageous to provide a vessel with a relatively small footprint, and the ability to handle solids as well as oil concentrations of greater than 300 ppm. Reducing the need for moving parts and chemical treatments, while allowing for retrofit of existing tanks, would also be desirable.